Tri-nuclear complex cyanine dyes are widely known and a number of them have been used as sensitizing dyes in photographic compositions. Such dyes can generally be represented by the formula: EQU A--L--B--L'--A',
in which A represents a nucleus of the cyanine dye type, B represents a cyclic ketomethylene residue, A' represents a nucleus of the cyanine dye type or a cyclic ketomethylene residue, and L and L' represent one or more methine linkages. Examples of such dyes are described in Hamer, Cyanine Dyes and Related Compounds, 1964, chapter XV. A number of cyclic ketomethylene residues have been utilized as B, such as: ##STR2##
Tri-nuclear dyes having such ketomethylene residues for B in the formula above may be prepared by a variety of methods known in the art. A common preparation scheme is exemplified as follows: ##STR3##
In such a reaction scheme, standard modifications include varying the length of the methine linkages (keeping the same odd/even number arrangement) and substituting S for the N--CH.sub.3 group in the center ketomethylene nucleus.
Known methods cannot, however, be used to prepare tri-nuclear dyes having a ##STR4## nucleus as the center ketomethylene nucleus. It is toward such dyes that the present invention is directed.